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Practical capacitors and inductors as used in electric circuits are not ideal components with only capacitance or inductance. However they can be treated, to a very good degree of approximation, as being ideal capacitors and inductors in series with a resistance; this resistance is defined as the equivalent series resistance (ESR). If not otherwise specified, the ESR is always an AC resistance measured with standardized frequencies.

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Electric circuit theory deals with ideal resistors, capacitors and inductors, each assumed to contribute only resistance, capacitance or inductance to the circuit. However, all components have a non-zero value of each of these parameters. In particular, all physical devices are constructed of materials with finite electrical resistance, so that physical components have some resistance in addition to their other properties. The physical origins of ESR depend on the device in question. One way to deal with these inherent resistances in circuit analysis, is to use a lumped element model to express each physical component as a combination of an ideal component and a small resistor in series, the ESR. The ESR can be measured and included in a component's datasheet. To some extent it can be calculated from the device properties.

Q factor, which is related to ESR and is sometimes a more convenient parameter than ESR to use in calculations of high-frequency non-ideal performance of real inductors, is quoted in inductor data sheets.

Capacitors, inductors, and resistors are usually designed to minimise other parameters. In many cases this can be done to a sufficient extent that parasitic capacitance and inductance of a resistor, for example, are so small as not to affect circuit operation. However, under some circumstances parasitics become important and even dominant.

Actual passive two-terminal components can be represented by some network of lumped and distributed ideal inductors, capacitors, and resistors, in the sense that the real component behaves as the network does. Some of the components of the equivalent circuit can vary with conditions, e.g., frequency and temperature.

If driven by a periodic sinewave (alternating current) the component will be characterised by its compleximpedanceZ(ω) = R + jX(ω); the impedance can involve several minor resistances, inductances and capacitances in addition to the main property. These small deviations from the ideal behavior of the device can become significant under certain conditions, typically high frequency, where the reactance of small capacitances and inductances can become a significant element of circuit operation. Models of lesser or greater complexity can be used, depending upon the accuracy required. For many purposes a simple model with an inductance or capacitance in series with an ESR is good enough.

These models, however simple or complex, can be inserted into a circuit to calculate performance. Computer tools are available for complex circuits; e.g., the SPICE program and its variants.

Pure capacitors and inductors do not dissipate energy; any process which dissipates energy must be treated as one or more resistors in the component model.

Inductors have resistance inherent in the metal conductor, quoted as DCR in datasheets. This metallic resistance is small for small inductance values (typically below 1 Ω). The DC resistance is an important parameter in switch-mode power supply design. It can be modeled as a resistor in series with the inductor, therefore often leading to the DC resistance being referred to as the ESR. Though this is not precisely correct usage, the unimportant elements of ESR are often neglected in circuit discussion, since it is rare that all elements of ESR are significant to a particular application.

An inductor using a core to increase inductance will have losses such as hysteresis and eddy current in the core. At high frequencies there are also additional losses in the windings due to proximity and skin effect. These are in addition to wire resistance, and lead to a higher ESR.

In a non-electrolytic capacitor and electrolytic capacitors with solid electrolyte the metallic resistance of the leads and electrodes and losses in the dielectric cause the ESR. Typically quoted values of ESR for ceramic capacitors are between 0.01 and 0.1 ohms. ESR of non-electrolytic capacitors tends to be fairly stable over time; for most purposes real non-electrolytic capacitors can be treated as ideal components.

Aluminium and tantalumelectrolytic capacitors with non solid electrolyte have much higher ESR values, up to several ohms, and ESR tends to increase with frequency due to effects of the electrolyte. A very serious problem, particularly with aluminium electrolytics, is that ESR increases over time with use; ESR can increase enough to cause circuit malfunction and even component damage,[1] although measured capacitance may remain within tolerance. While this happens with normal aging, high temperatures and large ripple current exacerbate the problem. In a circuit with significant ripple current, an increase in ESR will increase heat dissipation, thus accelerating aging.

Electrolytic capacitors rated for high-temperature operation and of higher quality than basic consumer-grade parts are less susceptible to become prematurely unusable due to ESR increase. A cheap electrolytic capacitor may be rated for a life of less than 1000 hours at 85°C (a year is about 9000 hours). Higher-grade parts are typically rated at a few thousand hours at maximum rated temperature, as can be seen from manufacturers' datasheets. Electrolytics of higher capacitance have lower ESR; if ESR is critical, specification of a part of larger capacitance than is otherwise required may be advantageous.

Polymer capacitors usually have lower ESR than wet-electrolytic of same value, and stable under varying temperature. Therefore polymer capacitors can handle higher ripple current. From about 2007 it became common for better-quality computer motherboards to use only polymer capacitors where wet electrolytics had been used previously.[2]

The ESR of capacitors of relatively high capacity (from about 1 μF), which are the ones likely to cause trouble, is easily measured in-circuit with an ESR meter.

^There is a very long discussion at [1] on component damage due to capacitor ESR increase. Essentially the switch-mode PSU of a particular PVR has a multisection transformer, each section being rectified and filtered for one of the voltages required. The PSU feedback is derived from the 5V supply; the drive adjusts to stabilise the 5V line. A problem that arises in practice is that the first electrolytic filter capacitor on the 5V line goes high-ESR. This reduces the voltage; feedback causes the drive to increase to maintain 5V. As the capacitor ages the effect worsens. If the other capacitors are OK, all voltages except 5V increase, until eventually even the 5V line cannot be maintained. Low voltages cause malfunction but usually not harm; but the excessive voltages on the other lines in this instance is enough to do quite a lot of damage to surface-mount semiconductors and disc drives. This is not a particularly exceptional case. This suggests that aluminium electrolytic capacitors shouldn't be used where they affect PSU feedback.